In AoW 3 you could level a unit as much as you wanted to, thou after reaching max rank it only gained 2HP per lvl beyond that.
But this was one of the most awesome thing about AoW. Taking good care off your units, and really work to keep them alive.
I see no such thing in AoW 4 sadly, and so far no mod that fixes it either...

Hope this is fixed soon!
And lastly, but not least, a great toast to all you beautiful modders out there! Keep up the werk, AoW 4 is turning into something truly divine...